The present invention relates to a method and a device for preparing a starch solution; a solution prepared in accordance with this method; and the use of such a solution for making paper.
In papermaking, cationic starch is widely used as a retention agent and strengthening agent. In recent years, cationic starch has to a large extent been employed together with anionic, inorganic materials, such as bentonite and various silica-based sols, to improve the retention and/or drainage in papermaking, see, for instance, EP Patent 41,056 and PCT Applications WO 86/00100 and WO 89/12661.
It has recently become possible to prepare high-cationized starch, i.e. starch with a degree of substitution exceeding 0.07. For instance EP Patent Specifications 233,336 and 303,039 disclose how this can be done by dry cationization, the latter disclosing the use of sodium aluminate as catalyst.
Generally, when starch is dissolved in water, the viscosity increases at first, and then decreases to a lower level. Thus, a viscosity maximum has to be passed, and this can, when dissolving low-cationized starch, only be achieved by thermal treatment at a temperature exceeding about 80.degree. C. For conventional dissolution of low-cationized starch, the starch is mixed with cold water in a slurry tank and subsequently boiled.
In the above-mentioned patent specifications, the high-cationized starch is said to be soluble in cold water. In cold-water dissolution, however, it is difficult to pass the viscosity maximum in a reasonable time and obtain a solution substantially free of unswelled starch grains, which is desirable if the solution is to give satisfactory results when used for making paper. Thus, cold-water dissolution necessitates long residence times while requiring a not too high dry content, for which reason the equipment used has to be dimensioned for large volumes.
To use heat during the dissolution process, in accordance with the method commonly employed for low-cationized starch, results in a high increase of viscosity in the slurry tank, which makes further handling of the solution more difficult. This increase is especially pronounced if sodium aluminate has been used as activator in the preparation of the starch, which thus contains aluminum.
It has therefore been necessary to maintain an extremely low dry content so that considerable amounts of energy are needed for the subsequent heating. Also in this case, the dissolution device including the heater has to be dimensioned for relatively large volumes, which leads to high costs.
Thus, it is desirable to reduce energy and investment costs by dissolving high-cationized starch during thermal treatment at high dry contents, preferably exceeding about 2% by weight.